1. Field of the Invention
The present invention relates to a reference voltage source circuit used in a bipolar monolithic integrated circuit such as an audio circuit and, more particularly, to a reference voltage source circuit used for determining a current bias of an amplifier including inverted-Darlington-connected transistors.
2. Description of the Related Art
FIG. 4 is a view showing a current bias circuit of a conventional amplifier including inverted-Darlington-connected transistors.
A reference voltage source circuit 21 and an amplifier 22 are connected to a power source terminal 20 receiving a voltage Vcc. In the reference voltage source circuit 21, one terminal of a current source Iref is connected to the power source terminal 20, and the other terminal of the current source Iref is connected to the collector and base of a diode-connected npn transistor Q1. The emitter of the transistor Q1 is grounded, and the base of the transistor Q1 is connected to the base of an npn transistor Q2 through a resistor Ri. The npn transistor Q2 constitutes the amplifier 22. A signal source vi is connected to the base of the npn transistor Q2 through a coupling capacitor Ci.
In the amplifier 22, the base and collector of a diode-connected transistor Q3 are connected to the collector of the npn transistor Q2, and the emitter of the transistor Q3 is connected to the power source terminal 20. The base of the transistor Q3 is connected to the base of a pnp transistor Q4. The emitter of the transistor Q4 is connected to the power source terminal 20, and the collector of the transistor Q4 is connected to an output terminal 23 and grounded through a resistor R.sub.L. The transistors Q1 and Q2, and the transistors Q3 and Q4 constitute current mirror circuits, respectively.
With the above arrangement, the electric characteristics of the transistors Q1 and Q2 are equal to each other, and the electric characteristics of the transistors Q3 and Q4 are equal to each other. When current gains of the transistors are relatively high, stable bias currents can be set.
An amplifier used as a current amplifier aiming at a low-voltage operation generally has a so-called inverted-Darlington arrangement in which the npn transistor Q2 and the pnp transistor Q4 are Darlington-connected to each other as described above. In addition, the transistor Q3 is connected to the transistor Q4 in consideration of a variation in current gain of the transistors used, thereby constituting a current mirror circuit.
When two transistors are Darlington-connected to each other, a current gain obtained by squaring a current gain of a single transistor can be obtained. However, as in the above circuit, when the transistor Q3 is connected between the transistors Q2 and Q4, the current gain cannot be increased.
FIG. 5 shows a prior art for increasing a current gain, and the reference numerals as in FIG. 4 denote the same parts in FIG. 5.
A resistor R is connected between the emitter of the transistor Q3 and a power source Vcc. A current flowing through the resistor R increases a current gain of the current mirror circuit constituted by the transistors Q3 and Q4. However, since input/output characteristics are not linear in this circuit, a distortion component is disadvantageously increased.